1. Field of the Invention
This invention relates to a thermal recording method and apparatus for use in a printing device such as such facsimile equipment or a digital copying machine and, more specifically, to an improvement in the method and apparatus for driving a plurality of heating elements of a thermal head simultaneously or dividedly according to the number of picture elements to be printed in one scanning line.
2. Description of the Prior Art
The heat-sensitive recording apparatus of the type referred to is conventionally arranged as shown, for example, in FIG. 3 to control the frequency of printing by the thermal head. More particularly, the system comprises a thermal head 10 having a plurality of heating elements corresponding to the number of picture elements in one scanning line, a paper feeding mechanism 20 including a motor driver 21 and a motor 22 for adjusting the feeding movement of papers (not shown) to be recorded by the thermal head 10, a pre-scaler 31 for detecting data to be printed, i.e., the so-called black data from picture information IF transmitted on the basis of a proper scale, a counter 32 for counting the number of the detected black data for each one scanning line of the picture information IF, a counter 40 for counting a clock signal CLK, and a ROM sequencer 51 which previously stores various sequence data concerning printing modes of the thermal head 10 and paper feeding modes of the paper feeding mechanism 20 based on the printing modes and which functions to receive addressing contents of the outputs of the counters 32 and 40 and sequentially send these data according to the addressing contents to the head 10 and mechanism 20 to control them. More concretely, the ROM sequencer 51, when addressed by the output of the counter 32 corresponding to upper 6 ones of 12 bits at its address terminals A6 to A11, designates the start address of a specific sequence to be selected depending on the proportion of black data to one scanning line data; whereas the sequencer 51, when addressed by the output of the counter 40 corresponding to lower 6 ones of the 12 bits at its address terminals A0 to A5, sequentially executes the selected sequence, whereby the number of printing times for one scanning line by the thermal head 10 is determined to provide a good efficiency according to the picture information contents. For this purpose, the thermal head 10 shown in FIG. 3 includes, in addition to the aforementioned heating elements, a shift register and a buffer memory for temporarily storing the transmitted picture information IF and a drive circuit for driving the heating elements on the basis of the stored information and the sequence data (strobe signals) applied from the sequencer 51.
In the aforementioned conventional thermal recording apparatus, if the allowable simultaneous printing of block data by the thermal head 10 is, for example, 25% or less of all the data of the picture information in one scanning line due to the temperature rise of the thermal head 10, the apparatus adopts two types of sequences, i.e., simultaneous printing and four-division printing to print the picture information of one scanning line, as shown in FIG. 4. In other words, if the count value of the counter 32 corresponding to one scanning line is less than 25% of the total number of picture data of the same line, then the strobe signals (indicative or sequence data D0 to D3) will be applied to strobe terminals ST0 to ST3 of the thermal head 10 simultaneously to designate the simultaneous printing mode. On the other hand, if the count value of the counter 32 is between 26 and 100% of the total picture data number, then the strobe signals will be applied to the strobe terminals ST0 to ST3 sequentially to designate the four-division printing mode.
Under the above conditions, when information data IF having the contents shown, for example, in FIG. 5 (a) is applied as the picture information of one scanning line, the thermal head 10 will be driven under control of the four-division printing sequence because the information IF contains 40% black data. When picture information IF having the contents shown in FIG. 5 (b) is applied as one scanning line picture information, the thermal head 10 will be similarly driven under control of the four-division printing sequence because the information IF contains 40% black data (shown as shadowed zones in FIG. 5) though such zones are dispersed.
Now attention will be directed to a picture information IF as shown in FIG. 5(b). When the thermal head 10 is driven under control of, for example, a two-division printing sequence, 20% black data of the information IF can be simultaneously printed. It will be noted that the printing speed of such a picture information IF by the two-division printing sequence can be higher than that by the four-division printing sequence. However, since the known apparatus cannot know the distribution of black data of the picture information IF, it is impossible to drive the thermal head with a good efficiency according to the contents or black data distribution of the picture information, which results in that the printing speed cannot be increased efficiently even though the apparatus previously detects the rate of black data in the information IF to determine a proper printing sequence for the thermal head.